The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A primary function of power supplies such as isolated AC-DC converters is to convert an AC signal from an AC signal source into a DC signal. The DC signal may be used to power a load. Another function of the isolated AC-DC converter is to provide isolation between the AC signal source and the load to prevent shock. The isolated AC-DC converter may also be designed to prevent damage to the AC-DC converter and the load due to high operating temperatures, short circuits, and other faults.
Control systems for isolated AC-DC converters typically include a transformer to provide isolation between the AC signal source and the load. A master control circuit is located on a primary side of the transformer. The primary side of the transformer is also directly connected to the AC signal source. The master control circuit provides a voltage control loop and a current control loop. The master control circuit may also provide protection functions such as monitoring of temperature, voltage and current limits.
An auxiliary control circuit is located on a secondary side of the transformer. The secondary side is directly connected to the load. The voltage control loop adjusts operation of the AC-DC converter such that a feedback voltage Vfb follows a reference voltage Vref. The current control loop adjusts operation of the AC-DC converter such that a feedback current Ifb follows a reference current Iref. The current feedback loop may also be based in part on the feedback voltage Vfb.
The feedback voltage Vfb is sensed on the secondary side of the AC-DC converter. The feedback voltage Vfb is fed back from the secondary side to the primary side through a coupler that provides isolation. The coupler may be an optical coupler or magnetic coupler.
Referring now to FIG. 1, an exemplary AC-DC converter 10 includes a primary side 12 and a secondary side 14. The primary side 12 includes an AC signal source 16 and a rectifier 18. For example, the rectifier 18 may be a full wave rectifier that includes diodes D1-D4 arranged as shown. An output of the rectifier 18 connects to a transformer 28 having a primary-side inductor 30 with Np turns and a secondary side inductor 32 with Ns turns.
The isolated AC-DC converter 10 is configured in a flyback converter topology. The transformer 28 is connected to a diode 34 arranged between one end of the secondary side inductor 32 and an output capacitor 36. The output capacitor 36 is connected across a load 40.
A control circuit 50 is located on and directly connected to the primary side 12 and includes a transistor 54 and a control module 56. The transistor 54 has a first terminal that connects to one end of the primary-side inductor 30. The transistor 54 selectively connects the primary-side inductor 30 to a reference potential such as ground. The number of turns on the primary side 12 (or Np) relative to the number of turns on the secondary side 14 (or Ns) is determined based on the ratio of the output voltage to the input voltage range.
The control module 56 receives a feedback voltage Vfb that is sensed on the secondary side 14 and that is output to the control module 56 via a coupler 60. The coupler 60 typically includes an optical coupler or a magnetic coupler.
The secondary side 14 may include a voltage divider including first and second resistors R1 and R2. The feedback voltage Vfb may be generated at a node between the resistors R1 and R2 and is fed back to the control module 56 via the coupler 60. A feedback current Ifb may be sensed by the control module 56 at a node A, which is connected to a resistor Rs and a terminal of the transistor 54. The control module 56 controls switching of the transistor 54 (via a control terminal of the transistor 54) to adjust the current and voltage supplied to the load 40.
When the transistor 54 is ON, magnetic flux increases in the transformer 28. A voltage across the secondary side inductor 32 is typically negative and therefore the diode 34 is reverse biased. Energy stored in the capacitor 36 flows to the load 40. When the transistor 54 is OFF, the diode 34 is forward biased and energy flows from the transformer 28 to the load 40 and the capacitor 36.
Some applications may require additional monitoring and control to be performed by the load 40. In these situations, the load 40 may need to send and receive additional control parameters and receive sensed parameters from the control module 56. To provide the additional information to the load 40 in the implementation of FIG. 1, additional isolation components are required for each additional signal that needs to be provided from the control module 56 to the load 40. The additional components needed to isolate these signals tend to significantly increase the overall size and cost of the electronic device.